Presently the disposal of industrial effluents and various other wastes causes environmental pollution, posing serious social problems and giving rise to the growing necessity of recycling these wastes using closed systems. Legislative efforts have resulted in to pollution control laws, while the (United Nations) Inter-Governmental Maritime Consultative Organization (IMCO) proposed International Convention for the Prevention of Pollution from Ships, 1973 including regulations for the disposal of sewage from ships. According to the Notification (No. 233) of the Japanese Ministry of Transportation, effluents from waste treating systems for ships must meet the following standards.
I. Biochemical oxygen demand (BOD):Below 50 ppm. PA1 Ii. Suspended solids (SS):Below 150 ppm. PA1 Iii. Number of coliform bacteria:Below 3,000/ml PA1 Iv. Solids over about 20 mm in size: None PA1 a. drainage and other wastes from any form of toilets, urinals, and WC scuppers; PA1 b. drainage from medical premises (dispensary, sick bay, etc.) via wash basins, wash tubs and scuppers located in such premises; PA1 c. drainage from spaces containing living animals; or PA1 d. other waste waters when mixed with the drainages defined above.
Until now, the most preferred method of waste treatment for ships is the activated sludge process which employs a fixed bed aeration tank. This method, however, generally reduces BOD to 60 to 100 ppm and SS to 100 to 150 ppm, at best and requires a flotation system in which an inorganic or organic coagulant is used, if it is desired to meet the above standards. Moreover, the method, rather than removing coliform bacteria permits the multiplication of the bacteria. The method has another serious drawback for marine services in that the apparatus used is heavy and requires a large space.